1. Field of the Invention
The present invention relates to a position measuring system which is suitable for generating position-dependent scanning signals, in particular for generating at least one reference pulse signal.
2. Discussion of Related Art
Besides incremental signals regarding the relative offset of two parts which are movable with respect to each other, known position measuring systems as a rule also provide reference pulse signals at one or several defined reference positions. With a defined relative position of the parts, which are movable with respect to each other and are connected with the scanning unit and the scale of the position measuring system, it is possible to provide an absolute reference by the position measurement which otherwise takes place relatively. The elements which are movable with respect to each other can be a workpiece, as well as a tool of a numerically-controlled machine tool, for example.
Regarding the arrangement of the reference markings on the side of the scale, there now exist a number of known possibilities. For example, they can be arranged on the scale laterally adjacent to the incremental track. However, with this variation it is problematic in principle that in case of a possible twisting, or de-adjustment, of the scale and the scanning unit around an axis parallel with the scale or scanning plane the exact assignment of the resulting reference pulse signal to a defined period of the incremental signals possibly is no longer assured at all. To overcome this problem it is known, for example from U.S. Pat. No. 3,985,448, the entire contents of which are incorporated herein by reference, to integrate one or several reference markings into the incremental graduation track on the scale. For this purpose it is possible, for example, to leave one or several lines out of the periodic incremental graduation track at the desired reference position along the measuring track in the incremental graduation.
Regarding the physical scanning principle, a differentiation is made in optical position measuring systems between systems with and without an optical collimation device in front of the light source used. If no optical collimation device is employed, this is generally called a divergent illumination. Such position measuring systems are advantageous in particular in regard to a compact total construction.
Solutions are today already known from the publications EP 0 887 625 A2 and WO 02/065061 A1 for generating a reference pulse signal in position measuring systems with divergent illumination, which results from the scanning of a reference marking which had been integrated into the incremental graduation track on the scale.
However, the variations for the generation of a reference pulse signal proposed in these publications have not been shown to be optimal in regard to as good as possible an efficiency of the reference pulse signal generation, low interference with the incremental signals, or the greatest possible elimination of the influence of the generated incremental signals on the reference pulse signal.